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Starvation

Starvation is defined as post-absorptive period –i.e. all food digested and no glucose coming in from gut We need to keep [glucose] blood ~5mM (>4mM) Under normal circumstances, brain can only use glucose –Cannot use FAs which cannot cross blood-brain barrier –So uses ~120 g glucose/day –Transported into brain cells by GLUT-1 Note that these are not insulin sensitive Although we store most of our energy as fat, we cannot convert FA into CHO –Acetyl CoA can’t be made into gluconeogenic precursors –Pyruvate  acetyl CoA is IRREVESIBLE

Glucose Requirements Parts of the kidney, skin and red blood cells have obligatory requirements for glucose –ie cannot use anything else but glucose Other tissues (such as Muscle and WAT) – can switch to fatty acids as an alternate fuel during starvation General strategy –Glucose conservation and recycling –De novo glucose formation

Hypo Danger zone! Liver Glycogen During the first few hours, the tissues are using glucose –So blood glucose concentration falls To prevent hypoglycemia, liver releases glucose into the bloodstream Thus [glucose] blood stays constant – or at least levels at ~4 mM Time (h) 240 Glucose (mM)

Glycogen Mobilisation - Glycogenolysis glycogen Glucose 1- phosphate Glucose 6- phosphate Glucose Phosphorylase GLUT-2 G6P Carrier GLUT-9 G6Pase G 6-P

Starvation - Muscle Muscle does not breakdown glycogen much in starvation because: –It has no glucagon receptors –It has no G6Pase,  cannot convert G6P  glucose  cannot release glucose into blood (only the liver has G6Pase) –However, some glucose residues in glycogen ARE released as neat glucose Because debranching enzyme uses water to hydrolyse the glycosidic linkages, not phosphate About 10% potentially released in this way Muscle is selfish with it’s glycogen!!

Glycogen Depletion Glycogen store in liver can supply glucose for brain < 24 hours Need to persuade other tissues to use fat rather than glucose Fat is stored in WAT (white adipose tissue)

Lipolysis Glucagon   [cAMP] –Also, lack of insulin contributes to this by slowing down the breakdown of cAMP  cAMP   activity of PKA PKA then phosphorylates HSL –Thus activating it PKA also phosphorylates perilipin –Perilipin is in the shell surrounding the fat vacuole –Allows the activated HSL to interact with the fat

Fatty acid oxidation Lipolysis releases FAs into the blood Note, GLUT-1 is still present in muscle –Even though a lack of insulin has led to GLUT-4s being endocytosed –So muscle potentially can still do glucose uptake Need to preserve glucose: –Get tissues to stop using glucose, and use FAs instead FAs will be oxidised to provide the acetyl CoA for the Krebs Cycle –But need to avoid oxidation of glucose, which is an irreversible reaction

PDH

Glucose-Fatty Acid Cycle In starvation we want PDH to be off –PDH kinase >> PDH phosphatase –PDH kinase is stimulated by acetyl-CoA –PDH is inactive when phosphorylated –Prevents wasteful oxidation of pyruvate –Pyruvate only made into lactate FA released from WAT (from lipolysis), causes [FA] blood to increase Uptake of FA into the muscle is also increased Oxidation of FA (  -oxidation) switches PDH off by producing a lot of acetyl CoA. This stop glucose oxidation

When PDH is off… Pyruvate cannot be oxidized to acetyl CoA –Then there is only one fate for pyruvate in the muscle, --- to be converted into lactate by LDH LDH = lactate dehydrogenase Lactate can be taken up by the liver –Made into glucose by gluconeogenesis Glucose recycling (glucose conservation) –Cori-cycle –Muscle Glucose  Pyruvate  lactate  liver  glucose (via gluconeogenesis)  glucose to the bloodstream again Gluconeogenesis can also happen from glycerol –Up to 30 g glucose per day can be made from glycerol

In Early Starvation…

Glucose Accounting Glycerol (from lipolysis) is the only source of DE NOVO gluconeogenesis –The lactate fuelled gluconeogenesis is just recycling –~30g glucose from glycerol per day But the brain needs ~120g/day, –not enough! –can brain glucose consumption be reduced?

Proteolysis Low insulin also leads to widespread proteolysis Amino acids could potentially be used for gluconeogensis –But often the amino acids are ‘burnt’ in the tissues –With the amine group coming out on pyruvate as alanine –Need to get the carbon skeletons to the liver

Glucogneogenesis Essentially a reversal of glycolysis Pyruvate  Glucose Requires three irreversible steps of glycolysis to be bypassed –Glucose ‘trapping’ The first step in glycolysis –Phosphofructokinase The rate limiting step in glycolysis –Pyruvate kinase The final step in glycolysis Gluconeogenesis can only occur in the liver –Mainly cytoplasmic

Gluconeognesis Requires ATP Stimulated in starvation –Only happens in liver Control steps illustrative of –Reversible phosphorylation –Allosteric activation –Gene expression Substrates include –Lactate Enters as pyruvate at the bottom –Glycerol Enters at aldolase stage (just as F16BP has split) –Amino acid carbon-skeletons Mainly supplied as alanine and glutamine

Glucose Accounting The brain needs ~120g/day, Substrates for gluconeogenesis –~30g glucose from glycerol per day –Glucose from lactate is just recycling –Alanine from muscle/tissue proteolysis Would need to provide 90 g/day Or 180 g protein per day, just for the brain Puts a huge strain on protein –can brain glucose consumption be reduced?

Proteolysis Hypoinsulinemia –Occurs when insulin level is really low Especially for a long period (>24 h) Proteins start to breakdown – PROTEOLYSIS Gives rise to amino acids Transamination reactions shuffle amine groups Channeled to the liver for gluconeogenesis as alanine/glutamine –Not all amino acids can be made into glucose Glucogenic - can be made into glucose Ketogenic - cannot be made into glucose –~2 g protein  1 g glucose

Fate of –NH 2 Amine groups are channeled into urea –Synthesised from aspartate and glutamate’s amine groups in the urea cycle Urea is non-toxic –The alternative would be conversion to ammonia, which is toxic Urea cycle only occurs in the liver

Lipolysis &  -Oxidation After ~2-3 days of starvation, the rate of lipolysis approaches a maximum –FA released into bloodstream  [FA] blood   –There is a limit to how fast muscles will use FA rate of  -oxidation depends on the demand of ATP by the muscles Regeneration of CoA by Krebs cycle needed to keep FA oxidation going BUT liver can do  -oxidation on FA even if there is no need for ATP –In the liver, CoA can be regenerated in a pathway other than the Krebs cycle

Ketone Bodies Ketone bodies – typically acetoacetate –Can be taken up & oxidised by the brain –Where they are split to 2 x acetyl CoA molecules –Tissues have to have mitochondria in order to use ketone bodies Ketone bodies reduce brain glucose use from 120g/day to 30g/day –all 30g could be provided by glycerol…. …. If it wasn’t for the use of glucose by the other carbohydrate-hungry tissues like skin, etc.

Extended Starvation After 2-3 days of starvation –Losses are g protein/day –Even though ketone bodies inhibit proteolysis and prevent protein being lost too rapidly Proteins are lost from all tissues –Although inactive muscles tend to slightly preferentially degraded –From heart, liver, brain, etc, as well  may cause severe damage to body Will reach equilibrium –where the amount of protein breakdown = the amount of glucose needed But the loss of body protein is ultimately what kills us